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Detection of a Hypercharge Axion in ATLAS Detection of a Hypercharge Axion in ATLAS a Monte-Carlo Simulation of a Pseudo-Scalar Particle (Hypercharge Axion) with Electroweak Interactions for the ATLAS Detector in the Large Hadron Collider at CERN Erik Elfgren [email protected] December, 2000 Division of Physics Lule˚aUniversity of Technology Lule˚a, SE-971 87, Sweden http://www.luth.se/depts/mt/fy/ Abstract This Master of Science thesis treats the hypercharge axion, which is a hy- pothetical pseudo-scalar particle with electroweak interactions. First, the theoretical context and the motivations for this study are discussed. In short, the hypercharge axion is introduced to explain the dominance of matter over antimatter in the universe and the existence of large-scale magnetic fields. Second, the phenomenological properties are analyzed and the distin- guishing marks are underlined. These are basically the products of photons and Z0swithhightransversemomentaandinvariantmassequaltothatof the axion. Third, the simulation is carried out with two photons producing the axion which decays into Z0s and/or photons. The event simulation is run through the simulator ATLFAST of ATLAS (A Toroidal Large Hadron Col- lider ApparatuS) at CERN. Finally, the characteristics of the axion decay are analyzed and the crite- ria for detection are presented. A study of the background is also included. The result is that for certain values of the axion mass and the mass scale (both in the order of a TeV), the hypercharge axion could be detected in ATLAS. Preface This is a Master of Science thesis at the Lule˚a University of Technology, Sweden. The research has been done at Universit´edeMontr´eal, Canada, under the supervision of Professor Georges Azuelos. My thesis receiver in Sweden has been Professor Sverker Fredriksson. The thesis is divided into three chapters. The first one treats the mo- tivations for the hypercharge axion and some theoretical background. The second covers the preparations and simulations as well as some predictions from the theory. The third chapter contains the results of the simulations and some conclusions. Notations and units: In the calculations we use units where Planck’s constant, the speed of light and Boltzmann’s constant are all equal to unity. A bar over a particle name signifies the antiparticle. The word axion is inter- changeable with hypercharge axion. Footnotes are used to explain further, but are not necessary for the basic comprehension. If a word is marked in slanted it is either supposed to be known, or is curiosum. These words are explained in the Glossary in Appendix A. All symbols, abbreviations and constants used in the thesis are listed and briefly explained in Appendix B. Finally I would like to express my deep gratitude to all the people that have helped on the different subjects of this thesis. First of all to my super- visor Georges Azuelos, Universit´edeMontr´eal, for his eternal patience with all my questions as well as numerous suggestions and good discussions. I would also like to thank Gilles Coutures, Universit´edeQu´ebec de Montr´eal, who has been a great help and who made the theoretical work leading to this thesis. I thank my professor in Sweden, Sverker Fredriksson, Lule˚aUniver- sity of Technology, for his support and all our previous discussions that have helped me in this work. As for the motivations for the hypercharge axion, I would like to thank Roger MacKenzie, Robert Brandenberger, James Cline, and Salman Habib for interesting discussions on cosmology, sphalerons and electroweak baryogenesis. Montr´eal in December 2000, Erik Elfgren. 1 Contents 1 Background and Motivations 4 1.1 Matter-Antimatter Asymmetry . .............. 5 1.1.1 How do we know that the Universe is Asymmetric? . 5 1.1.2 HowdidtheUniversebecomeAsymmetric?...... 5 1.2 Electroweak Model of Baryogenesis . .............. 8 1.2.1 Analogy with a Pendulum . .............. 8 1.2.2 Thermodynamic Nonequilibrium . .... 10 1.2.3 NonconservationofBaryonNumber.......... 10 1.2.4 ChargeandCharge-ParityViolation.......... 13 1.3TheAxion............................. 14 1.3.1 AmplificationofHypermagneticFields......... 14 1.3.2 TheLagrangian...................... 15 1.3.3 Couplings......................... 16 1.3.4 BranchingRatios..................... 17 1.3.5 Candidates for the Hypercharge Axion in Extensions oftheStandardModel.................. 18 2 Phenomenology of the Hypercharge Axion 19 2.1 The Detector . ..................... 19 2.1.1 Large Hadron Collider . .............. 19 2.1.2 The ATLAS detector . .............. 20 2.2 Characteristics of the Hypercharge Axion . .... 21 2.2.1 Interactions........................ 21 2.2.2 TheEvent......................... 22 2.3Approximations.......................... 24 2.3.1 MonteCarloSimulation................. 24 2.3.2 Weizs¨acker-Williams Approximation . .... 24 2.4Background............................ 25 2.4.1 DecayChannels...................... 25 2 2.4.2 γ Background....................... 26 2.4.3 Z0 Background...................... 27 2.4.4 JetBackground...................... 27 2.5Processes............................. 28 2.5.1 Process X γ + γ .................... 28 2.5.2 Process X → γ + Z0 γ + ¯l + l ............ 29 2.5.3 Otherprocesses......................→ → 29 3 Results and Analysis 31 3.1Signal............................... 31 3.2 Backgrounds . ..................... 33 3.2.1 Cuts............................ 33 3.2.2 Complete Background for m 1TeV........ 35 X ∼ 3.2.3 Significant Background Processes for mX 800 GeV . 38 3.3 Detection of the Hypercharge Axion in ATLAS .∼ . .... 39 3.3.1 Signal and Background for X γγ .......... 40 3.3.2 Signal and Background for X → γZ0 γ¯ll ...... 40 3.4DiscussionandConclusions...................→ → 42 A Glossary 45 B List of Symbols 48 B.1InEquations........................... 48 B.2ListofConstants......................... 49 B.3Abbreviations........................... 50 3 Introduction Ever since the discovery of antimatter it has been a mystery that almost all of our experiments in particle physics are symmetric in matter and antimatter, but yet the universe seems to be constituted entirely of matter. Some general conditions were outlined by Sakharov in 1967, but the problem itself remains unsolved. The first possible explanations were based on the grand unified theories (GUTs) in which the asymmetry was generated very close to big bang. These theories allow baryon-to-lepton decay, which could generate an asymmetry. One drawback is that the theories cannot be tested without tremendous 16 amounts of energy (& 10 GeV), far beyond our reach. Later on, theories evolved that could explain the generation of the asym- metry without GUT theories. Most of them suppose that the asymmetry was created around the electroweak phase transition at T 100 GeV when the electroweak symmetry was broken. Supersymmetric theories∼ can offer possible explanations under certain conditions, but the standard model itself seems incapable to produce the required asymmetry. A theory proposed by Brustein and Oaknin is particularly appealing for several reasons. It suggests simply that the introduction of a scalar field could create the required asymmetry through coupling to the hypercharge. As a bonus, the theory could also explain the existence of large magnetic fields in cosmic plasmas. The pseudoscalar was named hypercharge axion from its couplings to hypercharge. In summary, the hypercharge axion can possibly explain the dominance of matter over antimatter in the universe. In other words, why we exist! Finally, the hypercharge axion is expected to have a mass in the TeV range, making it possible to detect in the ATLAS detector at the large hadron collider (LHC) which is now constructed at CERN, Geneva, and will be operational by 2005. This is the subject of this masters thesis. Some references on particle physics are Nash [1], Peskin and Schroeder [2], and the CERN photo-gallery: http://press.web.cern.ch/Press/Photos/. 4 Chapter 1 Background and Motivations This chapter treats the background to my research, why it may be of interest to study the axion and what has been done so far. Briefly, the axion could be the reason for the mysterious domination of matter over antimatter in the universe. Before introducing the axion some theory is presented to situate it in its context and explain how the axion can be the reason behind the domination of matter. This chapter is divided into three major parts. The first one addresses some general questions, such as how we can be sure that the universe really has more matter than antimatter, and some very general conditions that must be fulfilled for the asymmetry to exist. These conditions are called the Sakharov criteria and they must be fulfilled for statistical reasons. The second part of this chapter is devoted to baryogenesis, i.e., the pro- cess by which the asymmetry is created. The subject is very complicated, so I will content myself with a rather qualitative description of the differ- ent phenomena. Among these are the electroweak phase transition,which 10 took place about 10− seconds after the big bang; the role of hypermag- netic fields in the phase transition; sphalerons which counter asymmetries and the Chern-Simons number, which is directly proportional to the baryon asymmetry. The third part treats the axion itself, with the properties proposed by Brustein and Oaknin [3]. A description of how the axion could amplify the hypermagnetic fields is included as well as the couplings and branching ratios. A discussion of where the hypercharge axion could appear is also provided.
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